The object of the present invention is a method and apparatus for drying a paper web or the like, as defined in the preambles of the independent claims presented below.
This means that the object of the present invention typically concerns a method and apparatus for drying a paper web or other similar web in the dryer section of a paper machine or the like, in which dryer section the web is dried against the heated cylinder surfaces of the drying cylinders and by means of air impingement drying with at least one air impingement unit.
Another object of the invention concerns the optimisation of paper web drying in the dryer section of a paper machine or the like, the dryer section comprising a drying section consisting of one or more drying cylinder groups and at least one air impingement unit. The invention is, however, intended to be applicable also to other types of dryer sections, if necessary.
It is previously known to use twin-wire web transfer and/or single-wire web transfer in the multicylinder dryers of a paper machine. During the past 15 years, single-wire web transfer has been used to an increasing extent, in which transfer there is only one dryer wire in each drying cylinder group, supported on which wire the web passes through the entire group so that the dryer wire presses the web with the help of the drying cylinders against the heated cylinder surfaces, the web remaining on the side of the outer curve on the turning cylinders or rolls. Thus, in single-wire web transfer, the drying cylinders are outside the wire loop and the turning cylinders or rolls are inside it. Previously known is the type of dryer sections which consist only of so-called normal single-wire web transfer groups in which the drying cylinders are in the top row and the turning cylinders or rolls in the bottom row.
In order to heat up the drying cylinders, steam is introduced inside them and the temperature of the drying cylinders is controlled by regulating the steam pressure and/or the rate of flow of the steam. It is also possible, although rarely implemented, to control the final moisture content reached in the dryer section by regulating the speed of the machine. In such a case the pressure of the steam is kept constantxe2x80x94in machines with limited drying capacity usually at maximum pressurexe2x80x94which means that the final moisture content can be increased or reduced by slowing down or speeding up the passage of the web through the dryer section.
The most commonly used steam pressure regulating systems are so-called cascade regulation and thermo-compressor regulation, which are described, for example, in the following publication: TAPPI NOTES, Practical Aspects of Pressing and Drying, Short Course, 1990.
One problem with these conventional-type dryer sections, where drying is carried out entirely by means of drying cylinders, by using either single-wire web transfer or twin-wire web transfer, has related to the regulation of drying efficiency. In order to achieve the desired final moisture content of the web, the drying efficiency of the drying cylinders is generally regulated by regulating the pressure of the steam supplied to the cylinders. This type of regulation is relatively slow and does not, therefore, react at optimum speed, for example, to sudden changes of moisture content in the web originating in the press section or wire section. Particularly in connection with a change of paper grade, start-up and web breaks, final regulation of drying to the optimum level by regulating the pressure of the steam is slow, due to the considerable mass of the drying cylinders. Methods used in regulation have included dry matter measurement based on IR measurement after the last drying cylinder, and feedback for controlling steam pressures, usually to the main steam cylinder group of the dryer section. This type of regulation has, as such, functioned without problems, where it has been a question of standard production at constant speed and no web breaks have occurred. Regulation problems occur, however, in connection with a change of paper grade, web breaks, or the paper machine start-up phase.
In connection with a change of paper grade, problems are caused in conventional dryer sections by the fact that each drying cylinder has a high thermal capacity, due to its great mass, which means that the temperature of the drying cylinders changes slowly. Because of this, the temperature changes in the drying cylinders have not been rapid enough where a change of grade is concerned. In some cases, the changes required relating to the regulation of drying efficiency have been made by changing the loads in the press section, but this will also change the qualities of the paper, which is obviously usually not desirable. Furthermore, when the loads in the press section are changed, the cross-direction profile of the paper web will also change, which means, for example, that there will often be defects in the moisture profile. Due to the foregoing reasons, a web of incorrect final moisture content or quality standard may be reeled on the reel-up in connection with a change of paper grade. According to solutions known from the prior art, it takes some 15-20 minutes after the change of grade before a balanced state is again reached. With paper machine speeds of, for example, 1500-1800 metres per minute, a large amount of paper, that is, paper of incorrect quality, is produced during this time. On a wide machine the amount may be 10-20 tonnes.
During a web break, on the other hand, problems arise, for example, due to the fact that the drying cylinders overheat when no paper that would transfer thermal energy away from the cylinders is supplied to the dryer section. Excessively hot cylinders cause problems in tail threading after a web break, as the tail threading cord adheres to the hot cylinders. In addition, excessively hot cylinders overdry the tail threading cord, which makes the cord brittle and causes it to lose its strength properties, which may cause problems in tail threading. Furthermore, at the stage when the tail threading cord broadens after a web break, it takes a long time before the drying cylinders return to an equilibrium temperature due to the drying cylinders"" slow ability to change their temperature, that is, their high thermal capacity. In case of a web break, there was previously no other way of bringing the situation under control than by reducing steam pressures for the duration of the web break. The result of this has in turn been that the final moisture content after a web break has not conformed to the desired values. Furthermore, it has taken a long time before the situation could be returned to correspond to normal running conditions.
At the paper machine start-up stage, the steam pressures suitable for a particular paper grade are usually first taken from a memory in which adjustment values that have proved good in earlier, corresponding running situations have been collected, for example, in tables, and the steam pressures in the drying cylinders are controlled by means of the above data. The steam pressures selected and their time staggering, or change, may also be based on computational models and values obtained thereby. Conventionally, when the web is first taken to the machine, the steam pressures used are slightly below the optimal pressures, and after this the steam pressures are increased to the desired level. The high thermal capacity due to the great mass of the drying cylinders makes start-up slow, so that it takes a long time before the desired situation is reached. This is problematic because during the start-up stage a large amount of paper of an incorrect type is produced.
The problems described above are thus mainly due to the fact that the thermal capacity of the drying cylinders is high and there is a long time-lag before they achieve the temperature changes required.
The dryer section is the part of the paper machine that uses the most energy. It can be said that as much as over two thirds of the energy consumption of a paper machine takes place in the dryer section. The dryer section should, therefore, be used as economically as possible, that is, in such a way as to achieve as high an evaporation efficiency as possible, and a high-quality drying result and low energy consumption. Drying must be uniform also in the cross direction of the web. Cylinder drying is currently the most common drying method used.
The web cannot be profiled, that is, evaporation cannot be regulated so as to be uniform in the cross direction of the web by means of regulating the steam pressure of the cylinders or the speed of the machine. In the dryer section, or even before it, streaks often form at some points on the web parallel to the travel direction of the web, in which the web dries more than at other points. Variations in the moisture profile of the web to be dried when it arrives at the air impingement unit are not only dependent on non-uniform drying in the actual dryer section, but are often due to non-uniform dewatering in the press section. Variations in the moisture profile may also be due to a non-uniform solid matter profile appearing already on the wire section. Changes in the need for drying also arise in connection with a change of grade. Moisture profile defects such as these must be rectified.
Previously, attempts have been made to rectify moisture profile defects by spraying water jets directed from jet pipes or nozzles fitted across the web at the said areas in order to render the web moisture level uniform. Adding water to the web is obviously less advantageous from the point of view of energy consumption, because the water sprayed for the purpose of regulation must be evaporated again from the web at a later stage. An alternative way of rectifying a moisture profile defect has been to use infrared dryers fitted across the web, which will evaporate water especially from the areas of the web with the highest moisture content. Infrared dryers consume relatively large amounts of energy.
In cylinder drying, the web to be dried is passed over the surface of the drying cylinder, pressed by the wire, which means that the side which is against the web cylinder at any time heats up and dries more efficiently than the other side of the web. In modern, fast machines, in which the web is dried using single-wire web transfer, only one and the same side of the web comes into contact with the cylinder surface in each dryer group, and thus dries more efficiently than the other side of the web. One-sided drying of the paper web is even further emphasised if the paper web is passed to the drying cylinders with the same side always in contact with them, also in the different drying cylinder groups. Paper dried in this one-sided manner tends to curl when in the form of sheets, which causes major problems in the finishing of the paper.
To solve the foregoing problem of one-sidedness, that is, curling, it has been suggested that a so-called reversed group be fitted in the dryer section, through which the web is passed supported on the wire so that the other, less efficiently dried side, will run in contact with the drying cylinders. This solution requires a different type of dryer section construction in which, deviating from other dryer groups, the drying cylinder and wire loop are fitted to run below the web. In this case, the paper broke formed during a web break or start-up falls down into the pockets formed by the wire loop, between the drying cylinders and the wire, from where it may be difficult to remove the broke. Due to the difficulties relating to the removal of broke, the runnability of this type of a reversed dryer section is poor in connection with web breaks and start-ups. In conventional single-wire web transfer, on the other hand, the wire loop is fitted to run above the web, in which case the broke formed in connection with a web break falls freely below the machine, from where it can easily be removed.
The aim of the present invention is in fact to achieve an improvement to the problems described above.
It is a particular aim of the invention to create a method and dryer section, in which rapid regulation of drying efficiency is possible, for example, in connection with a change of paper grade, web breaks and start-up situations.
Another aim is to achieve an energy-efficient method and apparatus which make possible rapid adjustment of overall evaporation, for example, in connection with a change of paper grade, a web break and start-up.
A further aim is to achieve a method and apparatus which make possible precisely targeted adjustment of evaporation, such as adjustment of profiling or evaporation also in the cross direction of the web.
Yet another aim is to achieve a method and apparatus, by means of which one-sided drying of the web and the consequent curling can be minimised.
Yet another aim is to achieve the type of method and apparatus for regulating the drying of a paper web which allows easy maintenance of the dryer section, rapid removal of broke and thus good runnability.
In order to achieve the above-mentioned aims, the method and apparatus relating to the invention are characterised by what is specified in the characterising parts of the independent claims presented below.
Instead of a dryer section based on conventional drying cylinder drying, the invention relates to a dryer section which applies both air impingement drying and drying by means of drying cylinders. As regards this type of dryer section, reference is made, for example, to the applicant""s Finnish patent applications FI 971713 and FI 971714.
In this application, air impingement drying refers both to air impingement drying, directed directly to the web, and to through-flow drying, being effected through the wire or a corresponding conveyor fabric. So-called through-flow drying, which is particularly well-suited for drying porous webs, is also included within the scope of the air impingement drying referred to in the invention. According to the invention, air impingement drying can be directed at a web passing over a large-diameter cylinder, roll, suction roll, through-flow cylinder or other curved surface. Air impingement drying can also be directed linearly, for example, to a web supported by a wire or belt which runs supported on rolls or blow boxes. The linearly running web may be arranged to run in a horizontal, vertical or inclined position. Hot air or superheated steam are preferably used as the medium in air impingement.
In order to achieve the aims presented in the foregoing and appearing later, a typical method relating to the invention for drying a paper web or the like in a dryer section comprising at least one air impingement unit, is characterised mainly in that, according to this method, the final moisture content of the paper web and/or any other machine-direction quality and/or the cross-direction profile are adjusted by regulating the efficiency of air impingement.
The apparatus relating to the invention for drying a paper web or the like in a dryer section comprising at least one air impingement unit is mainly characterised in that it incorporates a measuring instrument for measuring the final moisture content of the paper web and/or other machine-direction quality and/or the cross-direction profile, and means for regulating the blowing force of at least one air impingement unit on the basis of the measurement result.
According to the invention, air impingement drying can be used during different transient stages, such as those concerning a change of grade, a web break and start-up, for controlling changes in drying capacity, and for eliminating or at least minimising the problems occurring during these stages.
The advantages achieved by means of the invention are based especially on the fact that air impingement reacts extremely rapidly to adjustment measures, which means that it can be used for the rapid adjustment of drying efficiency required by, for example, a change of grade, a web break or start-up. In addition to the rapid adjustment of drying efficiency, the steam pressures of the drying cylinders in the dryer section are advantageously also regulated at the same time.
In the regulation of the drying efficiency of the dryer section and drying method relating to the invention, an optimisation algorithm known as such, may be used, which algorithm optimises the drying costs and/or qualities of the paper. According to the invention, for example, MPC (Model Predictive Control), that is, model predictive multivariable control can also be used.
When air impingement is used in accordance with the invention for regulating drying efficiency, one or more of the various air impingement parameters can optionally be adjusted as desired in the air impingement: for example, the blowing rate, the temperature of the blowing medium, the humidity of the blowing medium, and the distance of the air hood from the web (advantageous especially during web breaks). The air impingement hoods can also be built of machine-direction segments, in which case it will be possible to adjust and/or, where necessary, to close each segment separately. In order to adjust the cross-direction profile, the air impingement hoods can also be divided into cross-machine direction segments in which the above-mentioned air impingement parameters can be adjusted either together or separately.
In connection with a change of grade, the set values required by the new paper grade are usually known in advance, that is, for example, the steam pressures of the drying cylinders by means of which the desired end product is achieved. The steam pressure may thus be set at the desired level immediately at the start of the change of grade or gradually, also when applying the method relating to the invention. However, since this type of adjustment is slow, air impingement is regulated at the same time according to the invention, either on the basis of an existing air impingement model, by means of which the required regulation measures are calculated, or through continuous feedback regulation. As the drying efficiency of the cylinders gradually changes, the change is compensated by an opposite change in the drying efficiency of the air impingement.
At the stage of starting up a paper machine, the drying cylinders are first heated in accordance with known heating sequences. The air impingement hoods are preheated in a corresponding manner. After this, the running parameters may be set as desired in accordance with the predetermined values or the drying simulation calculation. By means of feedback, the values of the drying cylinders and air impingement can then be controlled so that the desired quality parameters are obtained.
When the running of a completely new paper grade is begun, the set values that have proved good for the closest paper grade among previous running situations are first selected from the memory, and after this, by utilising these set values and by applying feedback, air impingement and advantageously also steam pressures are regulated so that the desired values are obtained for the new paper grade.
In case of a web break, the steam pressures of the drying cylinders are dropped, the air impingement hoods are opened and the broke is directed for removal by means of the broke conveyors. The hoods"" own control system takes care of by-pass air circulation inside the hood, gas feed regulation, and regulation of the blowing rate, exhaust air and fresh air. When the track is restarted after tail threading, these measures are obviously carried out in reverse order and the required regulation of drying efficiency is carried out by means of air impingement.
The invention is naturally also applied in regulating drying efficiency during normal running, in addition to the special stages described above. According to the invention, the quality of the paper can be optimised continuously, also in the quality sense, while at the same time applying the cost data. This means that the position of the air impingement unit relating to the invention in the dryer section may also become a regulating parameter.
The position of air impingement has been taken into account in one particular embodiment of the invention, in which the dry matter content of a paper web coming from the press section of the paper machine and guided through at least one drying cylinder group, and dried to a dry matter content of preferably, for example, over 70%, even over 75%, is regulated by passing the paper web, supported on the wire or the like, through a slot-like space formed between
a curved or linear air impingement hood extending across one or more webs, and
a curved or linear surface, such as a cylinder, roll or vacuum box extending across one or more webs, and
by blowing towards the paper web, from the said one or more air impingement hoods in the said slot-like space, several successive hot air jets of predetermined drying efficiency and in cross direction to the web. If so desired, steam jets can be used instead of air jets, in which case, however, consideration must be given to the special demands made by hot steam on the hood construction of the hood as well as, for example, its sealing requirements.
The air impingement hood refers to any box-like construction known as such, from which hot air or steam jets are blown through holes, slots or other nozzles onto the web.
The vacuum box refers advantageously to a box-like construction creating a low vacuum of approximately 100-400 Pa, preferably 200-300 Pa, between the vacuum box and the wire/web, the side of the construction on the web""s side being mainly planar. The purpose of this relatively low vacuum is to prevent the detrimental detachment of the web from the wire. The aim is, by means of the vacuum, to prevent, the web from flapping, for example, due to the blowing from above, and thus coming into contact with the air impingement hood. The aim is to guide the web in a controlled manner through the slot formed between the boxes. The low vacuum required can preferably be created by means of a blow box, such as the one described in FIG. 4a below, or by means of a suction box.
A typical paper machine dryer section can be divided into three parts:
a first part, in which the paper web is primarily heated, however, at the same time increasing the dry matter content of the web typically within the range of 40-60%,
a second part, in which most of the free water in the web is removed through uniform evaporation in the drying cylinder groups through which the web is passed as single-wire or twin-wire web transfer, and in which the dry matter content of the web increases typically within the range of 45-85%, and
a third part, in which the web is finally dried by means of the drying cylinders, and in which the dry matter content of the web typically increases to a range of 75-98%.
In a special embodiment of the invention, the air impingement unit which enables the precisely targeted adjustment of drying can be fitted in the optimum area in the dryer section with a view to the regulation of drying and energy consumption, that is, for example, in an area where the web has already reached a dry matter content of  greater than 70%, preferably 75%. The said area is located at the end of the dryer section, typically before the last or second last drying cylinder group, a typical drying cylinder group in a dryer section provided with single-wire web transfer comprising approximately 3-8 drying cylinders.
The relative drying efficiency of the drying cylinders falls once the web has reached a dry matter content exceeding 70%, typically 75%, that is, when the major part of the readily evaporated water has been removed from the web. By means of air impingement, water bound more closely in the web can be removed, even from a web as dry as this, with good drying efficiency.
In the solution relating to the special embodiment of the invention, the new observation is utilised that by means of air impingement, as an efficient drying impulse,
it is possible to influence web drying advantageously, not only at the initial stages of drying, that is, while raising the temperature of the web, but expressly also at the final stages of drying. The use of the drying cylinders is most effective, from the energy efficiency perspective, more or less at the middle of the dryer section;
it is possible to influence the profiling of the web when the dry matter content of the web is more than 70%, even more than 90%, until the final dry matter content is reached. With previously known methods, attempts have been made to influence the profiling of the paper web at the end of the dryer section.
it is possible to eliminate the curling of paper in a preferred manner at the final stage in the dryer section, where the dry matter content of the web is already over 70%, typically over 75%, in an area after which the paper web no longer tends to dry one-sidedly as readily as in the middle of the dryer section. The use of reversed groups at the end of the dryer section has previously been suggested for the control of curling.
It has now been found that the optimum area for the combined raising of drying efficiency, profiling and curling control by means of air impingement is located at the end part of the dryer section, where the dry matter content of the paper web is more than 70%, preferably more than 75%, up to a dry matter content of about 95%, preferably within the range of 75%-85%. Even a very short, effective drying impulse lasting less than 1 second, or even less than 0.5 second, is often sufficient for regulating drying. A short, efficient drying impulse can be achieved by linear air impingement, over a length of 1-20 m, preferably 5-10 m.
Generally, it may be advantageous to carry out final drying after air impingement by means of drying cylinders, with one or two drying cylinder groups. In certain special cases, the air impingement unit relating to the invention can also be fitted at the very end of the dryer section. This should be done especially when the final dry matter content of the web being dried remains at 90% or only slightly above this.
The regulation of drying efficiency is usually based on the dry matter content of the web measured after the dryer section, irrespective of the cause of the need for a change in drying. Measurement may obviously also take place elsewhere, before or after the air impingement unit. The drying efficiency of the air impingement unit relating to the invention is also adjusted on the basis of the dry matter content measured. The drying efficiency of a typical air impingement unit relating to the invention is regulated by adjusting the temperature, moisture content or velocity of the air jets blown.
The blast air used in the air impingement unit is preferably blast air conducted from the paper machine room, or in a dryer section closed by a hood, the return air of the hood, or the air impingement device""s own return air. The temperature of the blast air is raised and/or its humidity level is lowered before blowing towards the paper web. The return air of the various air impingement units can be heated by means of a common burner, such as a gas or oil burner or other similar heater fitted in a separate space adjacent to the dryer section. On the other hand, an individual burner or the like may be integrated into each air impingement unit or part of a unit, in the web direction or in the cross direction of the web, which means that the different air impingement units or their parts can be adjusted independently of each other.
The air impingement hood or unit relating to the invention preferably blows hot air, the temperature of which is set between 40xc2x0 C.-500xc2x0 C., advantageously 200xc2x0 C.-400xc2x0 C., depending on the drying efficiency required at any time. The moisture content of the air jets typically varies between 0-300 g H2O/kg of dry air.
The drying efficiency of the air impingement unit can, however, also be regulated by adjusting the velocity of the air jets. In such a case the velocities of the air jets are typically maintained between 40-200 m/s, preferably between 50-150 m/s, and most preferably between 70-120 m/s.
By means of the efficient hot air or steam jet of the air impingement system relating to the invention, it is possible to regulate drying directed at the paper web extremely rapidly, practically without delay. The change of adjustment taking place in the air impingement hood is seen to its full extent in the paper web within a few seconds.
The temperature of the hot air can be regulated simply by adjusting the fuel valve of the burner. No time is taken for raising or lowering the temperature of the device itself, as in drying with a drying cylinder. By means of air impingement according to the invention, drying efficiency can be changed by 20-100% extremely rapidly. A complete change can typically be achieved in less than 30 seconds, usually in less than 10 seconds, which is only a fraction of the time required for bringing about the same change with conventional cylinder drying. Regulation by cylinder drying takes several minutes.
In the dryer section, two air impingement units can also be fitted in succession within the optimum range for regulating air impingement, in which case their drying efficiency can be adjusted either separately or together in order to achieve an optimal drying result. It is usually advantageous to fit successive air impingement hoods in the dryer section in such a way that the drying efficiency of the air jets coming from the first air impingement hood is on average greater than the drying efficiency of the air jets coming from the air impingement hood following it.
Each air impingement unit relating to the invention preferably comprises several adjacent rows of nozzles which are formed by blow nozzles fitted in succession across the web. These nozzles can be arranged so as to be adjustable all by the same adjustment, each nozzle separately, or a specified group of nozzles separately. It is often advantageous to divide the air impingement unit relating to the invention into several segments in the cross direction of the web, in which case the nozzles in the different segments can be adjusted separately. The segments may be as narrow as 100 mm. Typically, the width of the segments varies between 500 mm-2000 mm.